1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium, particularly to a process for producing a magnetic recording medium using a thin film of an improved ferrite as the recording medium.
The term ferrite as used herein denotes iron oxide of Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3. The iron oxide is crystallized directly on a substrate for supporting the recording medium, such as an anodized aluminum disc, so that the ferrite crystals are formed in a continuous film on the substrate. That is, the crystals are not separated from each other by any binder.
2. Description of the Prior Art
It is generally known that as a recording medium, in addition to a thin film of ferrite, a coating film including acicular, crystalline particles of .gamma.-Fe.sub.2 O.sub.3 dispersed in a binder or an electrolytically deposited film of a Ni-Co alloy can be used.
The necessary conditions required for a highly dense recording medium are as follows:
(a) The coercive force must be high:
(b) The squareness ratio of the curve B/H must be high.
(c) The film must be thin, so that the distance between a magnetic head and the recording medium is small.
(d) In the case of ferrite, both the reduction of .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4 and the oxidation of the thus produced Fe.sub.3 O.sub.4 to .gamma.-Fe.sub.2 O.sub.3 must be able to occur over a wide range of temperatures, so as to facilitate industrial production of both Fe.sub.3 O.sub.4 and .gamma.-Fe.sub.2 O.sub.3.
Referring to the above item (c), a coating film incorporating .gamma.-Fe.sub.2 O.sub.3 is polished in order to be made thinner. However, it is difficult to make the thickness of such a film thinner than 1 micron, because the crystalline particles of .gamma.-Fe.sub.2 O.sub.3 are dispersed in a binder. On the other hand, a magnetic recording medium of a Ni-Co alloy can be easily provided in the form of a thin film. However, this alloy is corrosive and exhibits poor wear resistance and, therefore, it is necessary to apply a protective film with a thickness of about 0.2 micron on the film of a Ni-Co alloy. Thus, both the coating film incorporating .gamma.-Fe.sub.2 O.sub.3 and the electrochemically deposited film of a Ni-Co alloy do not satisfy the required high recording density of a recording medium.
It is well-known that a continuous film of ferrite can be formed with a thickness sufficiently thin as to produce a highly dense recording medium, and that such a film is far superior in corrosion and wear resistances to either the coating film incorporating .gamma.-Fe.sub.2 O.sub.3 or the film of the Ni-Co alloy. The film of ferrite is generally produced by the following steps.
A continuous film of .alpha.-Fe.sub.2 O.sub.3 is formed on a substrate by using any of the procedures of the reactive sputtering process, i.e., chemical sputtering process, the vacuum vapor deposition process, the reactive vapor deposition process, the vapor phase growth process, the chemical deposition process and the like. The .alpha.-Fe.sub.2 O.sub.3 is then heated in a reducing atmosphere to produce Fe.sub.3 O.sub.4, which, if desired, is oxidized by heating in air to produce .gamma.-Fe.sub.2 O.sub.3. Thus, thin magnetic films of ferrite with a thickness of about 0.15 micron can be produced.
However, a process for producing a thin magnetic film of ferrite has the following difficulties.
One of the problems of the known films of ferrite used as a recording medium concerns the above item (d). That is, it is difficult to stably reduce .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4 and also to stably oxidize Fe.sub.3 O.sub.4 to .gamma.-Fe.sub.2 O.sub.3, due to their narrow ranges of heating temperatures for reduction and oxidation, respectively.
Other problems of the films of ferrite concern the above-mentioned items (a) and (b). That is, their magnetic properties are less than desirable for use as a highly dense recording medium, in which the coercive force H.sub.c and the squareness ratio are desired to be more than 500 Oe and 0.7, respectively.
Hereinafter, the above-mentioned problems will be described in detail.
The disc of anodized aluminum adapted for use in a highly dense magnetic recording medium generally has a relatively large diameter of 14 inches. Therefore, it is desirable that the magnetic properties of a thin film of ferrite reduced and oxidized on the disc not be affected by the differences in temperature which are apt to appear both between the portions of one disc and between discs during their heat treatment.
.alpha.-Fe.sub.2 O.sub.3 must be reduced at a temperature above the lower limit of the range of reducing temperature so as to be reduced completely to Fe.sub.3 O.sub.4, and also at a temperature below the higher limit of the range of reducing temperature in a controlled reducing atmosphere so that .alpha.-Fe.sub.2 O.sub.3 is not over-reduced to metallic iron.
The conventional Fe.sub.3 O.sub.4 containing no additional metals exhibits poor magnetic properties, such as 300 Oe of coercive force and 0.4 of squareness ratio when obtained at a temperature in the narrow optimum range of from 300.degree. to 325.degree. C. Further, such conventional Fe.sub.3 O.sub.4 is oxidized to a .gamma.-Fe.sub.2 O.sub.3 having 400 Oe of coercive force and 0.6 of squareness ratio only at a temperature very close to 230.degree. C.
A process of producing ferrite containing Cu is disclosed in U.S. patent application Ser. No. 773,963 filed on Mar. 3, 1977 by S. Hattori et al. In the case of ferrite containing Cu, the optimum reducing temperature can be shifted lower and broadened to a range of from 225.degree. to 300.degree. C. However, the coercive force of .gamma.-Fe.sub.2 O.sub.3 containing Cu still remains low at any oxidizing temperature.